power: Add error checking to MathExprPowerModel

power: Add error checking to MathExprPowerModel

MathExprPower model currently doesn't print any useful error messages
if an expression fails to evaluate. To add insult to injury, the model
only detects a failure when dumping stats and not at
initialization. This change adds a verification step in startup() that
ensures that all of the referenced stats actually exist.

Change-Id: I8f71c73341578d5882c8d93e482f5383fbda5f1d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2661
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>